An engine of a vehicle can operate in an idle condition during stationary or other related vehicle conditions. During cold start engine idle conditions, various systems may utilize waste engine heat to hasten engine warm-up, thereby enabling improved emission performance, engine efficiency, etc. As one example, waste exhaust heat may be adjusted to more rapidly increase catalyst temperature, thereby reducing emissions. Likewise, waste heat in the engine cooling system and/or lubricating system may be directed to the cabin for cabin heating or to the lubricating system, thereby reducing lubricant viscosity thus reducing friction. For example, spark timing may be retarded from MBT during initial starts to first heat an exhaust catalyst, and then once the catalyst is heated, spark timing may be advanced to before MBT to more rapidly heat engine coolant and/or lubricants to thereby provide increased engine efficiency.
One approach for adjusting engine operation during cold starting conditions is described in U.S. Pat. No. 6,334,431, which describes a method for utilizing spark timing advance past minimum spark advance for best torque (MBT) timing when the engine is under cold start conditions and after catalyst light-off to heat engine coolant. The advance value is based on engine coolant temperature, intake air temperature, engine speed, and manifold absolute pressure. Specifically, at engine speeds between 2000-2500 RPM, the spark timing advance is decreased as engine speed increases, and vice versa. Further, below 2000 RPM, the spark timing is independent of engine speed.
The inventors herein have recognized problems with the above approaches. As one example, at idling conditions when heating the engine coolant via advance timing past MBT, idle speed control may degrade. In particular, if spark timing is further advanced from an advanced timing relative to MBT in response to speed drops, the potential for engine stalls may increase. Further, if significantly advanced spark timing is desired to provide increased waste heat, combustion stability may decrease and, further, airflow increases may be insufficient to maintain sufficient engine torque to continue engine rotation at the desired idle speed.
The above issues are addressed by a method for controlling a vehicle engine having a plurality of cylinders and an electric motor configured to rotate the engine, comprising: during engine idling, advancing spark timing of at least one cylinder to substantially before a peak torque timing, and adjusting motor torque output of the electric motor to maintain engine idle speed.
By taking advantage of the torque reserve generated by the advanced spark timing used for increasing heat to engine coolant and/or lubricant, it is possible to improve idle speed control under such conditions. For example, by retarding spark timing in response to a drop in engine speed (while remaining advanced relative to MBT), it is possible to provide a rapid increase in engine torque, with only minor and likely temporary effects on heat delivered to the engine coolant and/or lubricant. As such, improved idle engine speed control can be achieved with reduced stalls while warming the engine. For example, the above approach can provide quick-acting torque reserve with the option of directing waste engine heat to the exhaust or the engine coolant, respectively. In this example, the directional choice of waste heat delivery can be achieved without impacting the level of torque and while maintaining torque reserve.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.